The present invention relates to a purge valve for controlling the flow of fuel vapor from a fuel vapor storage canister carried in a vehicle to the intake manifold of the vehicle engine.
To accommodate the withdrawal of fuel from the fuel tank of a vehicle by operation of the vehicle engine, the head space of the fuel tank must be vented to atmosphere. In the past, a simple vent line opening to atmosphere at a level high enough to prevent fuel from being vented was considered adequate, and no concern was given to the fact that under certain climatic conditions, and during refueling of the tank, a substantial amount of fuel vapor would be discharged from the head space of the tank into the atmosphere. In recent years, so-called on board vapor recovery systems have been produced in which the head space of the fuel tank is vented to a vapor storage canister filled with some vapor absorbent material, such as charcoal, and the resulting mixture of fuel vapor and air is conducted to the intake manifold of the engine for combustion. Because the canister can be purged only when the engine is running, the canister capacity and/or purge rate "schedule" must be such that the canister does not become over-saturated with fuel. This would allow liquid fuel to be conducted to the engine and/or allow fuel vapors to escape to atmosphere. For a given system's canister capacity constraints, the required purge rate could actually become high enough to satisfy an engine's fuel requirements at idle.
It is thus necessary to control or throttle (i.e. modulate) the flow of fuel vapor from a vapor storage canister to the intake manifold of the vehicle engine to avoid over- or under- enriching the fuel mixture supplied to the manifold by the fuel injection system. Purge valves for this purpose which are controlled by the electronic control unit now conventionally employed to control many vehicle operation functions have been developed in the prior art.
Typically, these purge valves include a solenoid actuated on-off valve in the line connecting the vapor storage canister to the manifold. The valve solenoid is cyclicly energized and de-energized by a pulse width modulated control signal generated by the vehicle's electronic control unit in accordance with various engine operating parameters sensed or monitored by the control unit. This type of control is frequently referred to as a duty cycle operation in which the signal received from the control unit opens the valve for a regulated portion of a cycle and closes the valve for the remaining portion of the cycle. The pressure inducing flow of vapor while the valve is open is the pressure differential between substantially atmospheric pressure (the vapor storage canister is vented to atmosphere) and manifold vacuum, which can vary substantially, dependent upon the vehicle engine operating conditions. In U.S. Pat. No. 4,944,276, a purge valve employing a duty cycle operated solenoid valve and a vacuum regulator which establishes a constant flow inducing pressure differential is disclosed.
Duty cycle operation of an on-off valve assumes that the total flow through a valve which is open half the time will be equal to the total flow through the same valve if the valve is held halfway open over an equal time period. This assumption, however, does not take into account the fact that flow through the valve when continuously halfway open is continuous flow at a constant rate, while flow through the valve in a duty cycle operation is a pulsating flow. Tests have shown that when the fuel vapor in the canister is a relatively high mixture, the pulsating vapor flow to the intake manifold under the control of a duty cycle operated on-off valve will generate correspondingly pulsating peaks in an exhaust emission analyzer.
The present invention is directed to a duty cycle controlled purge valve which meters a steady flow of vapor/air from a vapor storage canister to the engine intake manifold at a continuously variable rate.